© 2009
Custom Sensor
Solutions, Inc.
Making Accurate Vapor Dilutions for Calibrating Gas Analyzing Instruments
(continued - Page 2)
Go to page 1  page 3

Standard Gases in Cylinders

Vapors of many chemicals can be ordered ready-made and certified from specialty gas suppliers such as Matheson or Scott Specialty Gases. These standard gases are generally prepared gravimetrically and their accuracy is certified. Nothing can be more convenient, but the cost is high. A cylinder of standard gas may cost $200 to $500, not counting the cost of special regulators that are often required. Also, gas cylinders cannot be shipped by air. This, together with local regulations and prudent safety considerations, restricts the number of different gases and concentrations that can be stored in a laboratory.

There are certain limitations on the availability of standard gases, which should be discussed with a supplier before ordering.

1. Some chemicals are too corrosive or too unstable to be made into standard gases.

2. There is usually a limit on the concentrations that are available. Above a certain concentration, many liquid chemicals will condense from the gas phase at the high pressures found inside the cylinder.

3. Nearly all standard gases in cylinders have a limited lifetime.

4. Read the specs. on the gas carefully. "Zero" grade compressed air can still contain up to 1 ppm of CO.

Permeation Tubes

Permeation tubes are a nearly ideal way of making large volumes of vapor from certain chemicals at low concentrations. They are made by sealing volatile chemicals in tubes, usually of a fluorinated polyethylene. The chemicals will permeate (or "leak") through the walls of the tube at a rate that is controlled by the temperature.

The permeation tube is typically enclosed in a thermostated glass chamber. The chamber is flooded with a controlled flowrate of the diluting gas. The constant leakage of chemical into the constant flow of gas produces a constant concentration. Special ovens and apparatus are sold for heating and using permeation tubes. If you have an water bath with accurate temperature control and a good flowmeter, it is possible to construct an apparatus cheaply.

The concentration can be varied by changing the flowrate of the carrier gas, or by changing the temperature of the chamber. The flowrate is the "fine control", allowing variation of concentration over a two- to five-fold range. Temperature is the "coarse control"; a 10 degree change will cause a four- to ten-fold change in permeation rate, depending on the chemical and the tube material.

Several manufacturers make permeation tubes. The tubes may cost from $20 to $200 and most will last for months or years. They are usually supplied with a certificate of calibration, but they can be calibrated by any user with a five-place analytical balance. The history of the tube is kept in a logbook, so that its weight loss over a period of operation at a specific temperature can be used to confirm its calibration.

Permeation tubes are available for a wide variety of chemicals. They work best for liquids with low boiling points and for gases that are liquified at low pressures and room temperature. Hard-to-handle chemicals like nitrogen dioxide, ethyl mercaptan, and acetaldehyde are ideal for permeation tubes. It is well to remember, however, that oxygen leaks into a permeation tube as quickly as the chemical leaks out. Easily oxidized compounds like thioethers and mercaptans will eventually become contaminated with oxidation products.

Permeation tubes are a safety hazard if they are damaged or stored elsewhere than in a fume hood. The vapor continues to leak even at room temperature for most compounds, and if they are stored in a tight container, the pressure may build up as the compound permeates outward, causing a sudden release when the container is opened. Potentially, the container may burst, but we have never heard of this happening.

Chemical Generation

Some gases are more easily made as they are used, rather than stored. Examples are ozone, hydrogen chloride, and hydrogen bromide. Low concentrations of ozone can be made with devices that pass ordinary air across a powerful ultraviolet lamp. A calibrated source that we use in our laboratory is the Thermo Environmental Co. Model 565 Ozone Generator, which will produce 2 liters/min of 0.5 PPM ozone. A dial on the front panel will vary the ozone concentration continuously over the range 0 to 500 PPB.

Hydrogen chloride can be made by injecting a measured amount of concentrated sodium chloride solution into a flask containing concentrated sulfuric acid. All the generated hydrochloric acid will be converted to HCL gas, which can be swept out of the container with a stream of nitrogen. A simple way to do this is to use a 60 cc serum bottle. Put 10 mL sulfuric acid into the bottle and close it with a rubber serum cap. Heavy-gauge (16 or 18) hypodermic needles can be pushed through the serum cap and connected as inlet and outlet for the nitrogen. The hydrochloric acid is injected (slowly!) through the serum cap with a Teflon or plastic disposable syringe and another stainless steel needle. (The needle will be corroded by HCl and should be washed, cut in two and discarded immediately.) Flush the gas generated into a sample bag with a measured volume of nitrogen using the large syringe or stopwatch-and-flowmeter method described below.

Caution: Dispose of used sulfuric acid solutions properly.

Go to page 3

Back to Apnotes Index